Formation
Subtropical cyclones form in a wide band of latitude, mainly south of the 50th parallel in the northern hemisphere. Due to the increased frequency of cyclones which cut off from the main belt of the westerlies during the summer and fall, subtropical cyclones are significantly more frequent across the North Atlantic than the northwestern Pacific ocean. In the eastern half of the north Pacific ocean and north Indian ocean, the older subtropical cyclone definition term is still used, which requires a weak circulation forming underneath a mid to upper-tropospheric low which has cut off from the main belt of the westerlies during the cold season (winter). In the case of the north Indian ocean, the formation of this type of vortex leads to the onset of monsoon rains during the wet season. In the southern hemisphere, subtropical cyclones are regularly observed across southern portions of the Mozambique Channel.
Most subtropical cyclones form when a deep cold-core extratropical cyclone drops down into the subtropics. The system becomes blocked by a high latitude ridge, and eventually sheds its frontal boundaries as its source of cool and dry air from the high latitudes diverts away from the system. Temperature differences between the 500 hPa pressure level, or 5,900 meters (19,400 ft) above ground level, and the sea surface temperatures initially exceed the dry adiabatic lapse rate, which causes an initial round of thunderstorms to form at a distance east of the center. Due to the initial cold temperatures aloft, sea surface temperatures usually need to reach at least 20 °C (68 °F) for this initial round of thunderstorms. The initial thunderstorm activity moistens up the environment around the low, which destabilizes the atmosphere by reducing the lapse rate needed for convection. When the next shortwave or upper level jet streak (wind maximum within the jet stream) moves nearby, convection reignites closer to the center and the system develops into a true subtropical cyclone. The average sea surface temperature that helps lead to subtropical cyclogenesis is 24 °C (75 °F). If the thunderstorm activity becomes deep and persistent, allowing its initial low level warm core to deepen, tropical cyclogenesis is possible.
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